CHAPTER 6

Projection Geometry

IMAGE SHARPNESS AND RESOLUTION Sharpness: how well a boundary between two areas of

differing radiodensity is revealed.

Image spatial resolution: how well a radiograph is able to reveal small objects that are close together.

x rays are produced at the target in an x-ray tube

originate from different points and travel in straight lines,

their projections of a feature of an object do not occur at exactly the same location on an image receptor.

image of the edge of an object is slightly blurred rather than sharp

There are three means to maximize image sharpness:

1. Use as small an effective focal spot as practical.

2. Increase the distance between the focal spot and the object by using a long, open-ended cylinder.

3. Minimize the distance between the object and the image receptor.

IMAGE SIZE DISTORTION Increasing the focal spot-to-image receptor distance

and decreasing the object-to-image receptor distance minimizes image magnification.

The use of a long, open-ended cylinder as an aiming device on an x-ray machine thus reduces the magnification of images on a periapical view.

IMAGE SHAPE DISTORTION This situation arises when not all the parts of an

object are at the same focal spot-to-object distance.

Such a phenomenon is seen by the differences in appearance of the image on a radiograph compared with the true shape.

There are two means to minimize shape distortion:

1. Position the image receptor parallel to the long axis of the object.

A) the central ray of the x-ray beam is perpendicular to the image receptor, but the object is not parallel to the image receptor. This type of shape distortion is

called fore-shortening

B) the x-ray beam is oriented at right angles to the object but not to the image

receptor; this results in elongation

2. Orient the central ray perpendicular to the object and image receptor.

Image shape distortion occurs if the object and image receptor are parallel, but the central ray is not directed at right angles to each. This distortion is most evident on maxillary

molar views

BISECTING-ANGLE TECHNIQUES

PARALLELING-ANGLE TECHNIQUES

OBJECT LOCALIZATION the dentist may wish to use radiographs to determine the

location of a foreign object or an impacted tooth within the jaw.

Three methods are frequently used to obtain such three-dimensional information.

The first is to examine two images projected at right angles to each other .

The second method is to use the tube-shift technique employing conventional periapical views .

Third, in recent years, the advent of cone-beam imaging has provided a new tool for obtaining three-dimensional information.

The position of the maxillary zygomatic process in  relation to the roots of the molars can help in identifying the orientation of views.

EGGSHELL EFFECT The top photon has a tangential path through the apex of

the egg and a much longer path through the shell of the egg

lower photon, which strikes the egg at right angles to the surface and travels through two thicknesses of the shell.

expanded cortex The periphery of the expanded cortex is more opaque

than the region inside the expanded border.

The cortical bone is not thicker on the cortex than over the rest of the lesion, but rather the x-ray beam is more attenuated in this region because of the longer path length of photons through the bony cortex on the periphery.

expanded cortex